The invention relates to fork-lift application of hydraulic-lift systems of the variety disclosed in my copending application Ser. Nos. 570,590, filed Jan. 13, 1984, now abandoned and Ser. No. 601,481, filed Apr. 18, 1984, now U.S. Pat. No. 4,715,180, which applications are incorporated by reference. More particularly, the invention relates to battery-operated electric-motor driven hydraulic means for selective intermittent up/down manipulation of the lift platform of a fork-lift vehicle.
In conventional fork-lift vehicles which rely upon electric-motor drive of a hydraulic-lift system, the electric motor and its battery are designed and configured with capacity to sustain maximum loads. The motor drives a fixed-delivery hydraulic pump, and therefore the current drain on the battery, at start of a load-lifting procedure, can be as much as three or more times the current consumption for normal running. The conventional motor is series-wound, operating fast at low loads and slow at high loads, in a very inefficient manner. When approaching a target elevation in an upward stroke, speed is controlled by restricting flow to the hoisting cylinder, the remainder of the flow being discharged at maximum pressure to sump, via a relief valve; further discharge to sump is involved in the course of a downward stroke. Not only are such convenventional systems grossly wasteful of energy, but they are unnecessarily damaging to the battery, in that large surges of current drain are a fact of life, resulting in need for frequent recharging and in accelerated destruction of the battery; beyond the magnitude of the surge, heavy current surges have a negative effect on battery life, due to the chemical nature of the battery action and to the lower efficiency which necessarily accompanies such surges. Stored battery energy must do all the lifting, and such potential energy as is available after lifting a load is discarded, by release of hydraulic fluid to sump, for a controlled descent of the loaded fork.